1. Field of The Invention
This invention relates to optical information processing and, more particularly, to recursive optical filtering means in which recursion is accomplished in the Fourier plane of the optical filter to reduce the throw of the optical train.
2. Discussion Of The Prior Art
In selective spatial frequency filtering, there are significant advantages in accomplishing the filtering function optically instead of by use of the more usual electronic filter, particularly if a number of filters are desired. Optical spatial filtering in the prior art is typically accomplished with filtering systems having a single stage. In those single-stage prior art systems, particularly those using a programmable spatial filter (PSF), the maximum filter attenuation is about 25 dB, which is the maximum attenuation realizable with present1y available components with a single-stage PSF.
In the prior art, R. W. Brandstetter, A. R. Doucette, and C. E. Lindig disclose an optical filtering system in which the radiation being processed is caused to make multiple passes through the filter to provide a significant improvement in the attenuation achievable with the filter. In that prior art system, disclosed in patent application Ser. No. 498,462, filed May 26, 1983, which matured into U.S. Pat. No. 4,522,466 and assigned to the assignee in the present patent application, the PSF stages are effectively cascaded by means of recursions and the attentuation achieved thereby can be increased as a function of the number of stages or recursions.